Is your gas hot water heater back drafting?

This article pertains to atmospheric vented gas water heaters and a warning sign that could indicate that the heater may be back drafting potentially toxic gases the wrong way: into your living space. An atmospheric vented water heater is shown in the picture below. There is an air space between the top of the water heater and the vent pipe that is supposed to carry the combustion gases out of the home. If you have a power vented gas water heater, also shown below, then this article will not apply.

Power Vented Gas Water Heater - the black box at the top is a fan that closes the air gap and pulls gases out.

If you look closely at the top of your water heater and you see debris as shown in the example below, this could be a warning that the combustion biproducts are coming back into your home, pulling debris with it as it comes down rather than up the vent.

Close-up of debris at top of water heater

This example is clearly not just dust or debris from the home, but coming from inside the exhaust pipe from that air gap. Notice how the particulate is denser nearest the pipe.

If you do not have a carbon monoxide detector in the general area of your gas water heater, then you will definitely want to do that. However, it is possible to get short bursts of these contaminants or lower levels than a typical detector is designed to alarm at. Without going into any more detail on this, if interested, check out the blog article on my website titled “Why you should upgrade to a low level carbon monoxide alarm.”

To test the theory that the debris on top of your water heater is from back drafting of combustion gases, here is a little test you can do…

Wait until the heater has been off for a while and the vent has cooled. Turn on the exhaust fans in your bathrooms and kitchen range hood, as applicable. This creates more of a worst case condition of depressurizing the home). Now, find a small piece of clear glass, plexiglass, mirror or even a pair of sunglasses. Have someone turn on the hot water in the home at a nearby sink as you hold the mirror/glass vertically near the air gap as illustrated below:

As the hot water runs, you should hear the gas flame kick in as gases will start coming up and out of the heater and into the vent pipe at the top. Watch for condensation on the mirror/glass as you slowly move it around all sides of the gap. If combustion gases are spilling out of that air gap, they will also carry moisture creating condensation on your mirror/glass. If this only happens momentarily but corrects itself within a minute, then it may not be a legitimate issue (some standards allow spillage up to 1 minute). The closer you get to 1 minute of continuous back drafting/spillage, the more you should suspect a possible issue. The potential causes are beyond the scope of this article, but this test could be enough to identify a potential issue and know that you should have a qualified person like a plumber or HVAC professional take a closer look.